code: def backward(self, indexes, gy):
      x = self.get_retained_inputs()
      gx = sinh(x[0])
      gx *= gy[0]
      return (gx,)
  
file_path: chainer/chainer/functions/math/hyperbolic.py
imports: from chainer import backend
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 4 for calling the Cosh obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: gx